A Level Biology Practice Exam

What characterizes alpha-glucose?

1. The hydrogen atom on carbon no.1 projects below the plane of the ring

2. The hydrogen atom on carbon no.1 projects above the plane of the ring

3. It has a higher energy content than beta-glucose

4. It is a disaccharide form of glucose

The correct answer is: The hydrogen atom on carbon no.1 projects above the plane of the ring

Alpha-glucose is characterized by the orientation of the hydroxyl group attached to carbon 1 in its cyclic form. In alpha-glucose, the hydroxyl group on carbon 1 is oriented downward, which means that the hydrogen atom at this carbon projects upwards when visualizing the six-membered ring structure. This distinct configuration differentiates alpha-glucose from beta-glucose, where the hydroxyl group on carbon 1 is positioned upwards, causing the hydrogen atom to project downwards. This structural difference is crucial because it affects the properties and reactivity of the two forms of glucose, especially in the formation of polysaccharides. For instance, starch is made from alpha-glucose units, whereas cellulose is made from beta-glucose. This reflects how the structural characteristics of a molecule can dictate its biological role and function. Understanding the stereochemistry of glucose molecules is foundational to grasping more complex carbohydrate chemistry and metabolism, making this characteristic pivotal in various biological contexts.